Various synthetic fibres are known and used in the field of non-wovens for the preparation of non-woven fabrics for a variety of purposes, in particular various polyolefins and polyolefin derivatives, e.g. polypropylene and polyethylene. However, for the purpose of non-woven materials for use in the medical industry both polypropylene fibres and polyethylene fibres suffer from disadvantages which until now have limited the extent of their use. It has furthermore proved difficult to produce non-wovens which have a soft feel resembling that of natural materials, e.g. for use in baby diapers and feminine hygiene products.
GB 2 121 423 A discloses hot-melt adhesive fibres comprising a polyethylene resin composition alone, consisting of 50-100% by weight of polyethylene with a density of 0.910-0.940 g/cm.sup.3 and a Q value (Q=M.sub.w /M.sub.n) of 4.0 or less and up to 50% by weight of a polyethylene with a density of 0.910-0.930 g/cm.sup.3 and a Q value of 7.0 or more, and composite fibres in which the above composition is one of the composite components and forms at least a part of the fibre surface. U.S. Pat. No. 4,522,868 discloses neutron shielding sheath-and-core type composite fibres in which the sheath and core components may be composed of polyethylene or polyethylene copolymers, the core component comprising at least 5% by weight of neutron shielding particles. The fibres are designed for use in neutron shielding fabrics due to the presence of a large amount (preferably 10-60% by weight in the core component) of the neutron shielding particles. The fibres of the present invention, on the other hand, which are adapted for use in various thermally bonded non-woven medical and hygienic products, and not specially adapted for neutron shielding fabrics, need not contain such neutron shielding particles.
It is necessary that non-woven materials which are to be used for medical purposes can be sterilized, this sterilization typically being carried out using radiation, e.g. in the form of .gamma.-radiation or .beta.-radiation. However, polypropylene materials are damaged by such radiation treatments. Even fibres prepared from polypropylene materials which have been stabilized --so-called "radiation resistent" polypropylene --will be damaged at high dosages, because of the very large specific surface area of the fibres (typically about 50-100 m.sup.2 /kg). Polypropylene's lack of ability to withstand radiation is also seen in bicomponent fibres with a polypropylene core and a sheath of e.g. polyethylene. The effect of radiation on polypropylene is due to the fact that the radiation produces chain scission at the tertiary carbon atoms of the polypropylene molecules. Polyethylene, on the other hand, does not have these tertiary carbon atoms, and is therefore not nearly as susceptible to such radiation. In addition, polyethylene has the ability to form cross-linkages, a property which polypropylene does not have.
Polyethylene is thus able to tolerate the radiation treatments used to sterilize medical products, but known polyethylene fibres also suffer disadvantages which until now have limited the extent of their use. Thus, the use of linear low density polyethylene (LLDPE) has been limited by the fact that it has not been possible to use a high stretch ratio during the preparation of LLDPE fibres, and, more importantly, by the fact that it has not been possible to provide LLDPE fibres with a permanent texturization. As a result, such fibres are unsuitable for the preparation of most types of non-wovens, as the carding processes used for the preparation on non-wovens require that the fibres have a certain texturization. Only non-wovens produced by processes other than carding and thermal bonding can be made with LLDPE fibres. Fibres of high density polyethylene (HDPE), on the other hand, may be provided with a permanent texturization and may be stretched during processing using a high stretch ratio, but HDPE fibres are stiff and therefore unsuitable for non-woven materials in which a soft feel is necessary.
In addition, monocomponent fibres of either LLDPE and HDPE alone are generally unsuitable for thermobonding due to the fact that they have a very narrow "bonding window" (i.e. a narrow temperature range in which they may be thermobonded), thereby making it difficult to adequately control the thermobonding process within the required temperature range. This narrow bonding window is due to the fact that such monocomponent fibres must be softened during thermobonding, but must not melt if they are to contribute to the structure of the article in which they are used.
It has now been found that these problems may be avoided by preparing non-woven fabrics, e.g. for medical use, using thermobondable bicomponent synthetic fibres comprising two different types of polyethylene. It is thus possible according to the present invention to prepare non-woven fabrics using novel fibres which maintain their texturization during processing and therefore are suitable for carding, which have a broad bonding window and therefore are suitable for thermobonding, and which are able to tolerate the .gamma.- and .beta.-radiation used to sterilize medical products. The fibres furthermore have a soft feel and are therefore suitable for the preparation of non-woven materials in which softness is required or desired, e.g. various hygienic products such as coverstock for baby diapers, feminine hygiene products, etc., as well as non-woven materials for medical use.